The purpose of this work is to examine proteins containing specific residues enriched with respect to C13. To attain the desired specificity and to use the enrichment to guide the synthesis the strategy is to employ chemical methods for semi-synthesis. In this fragments of the natural proteins are used for the great part of the final structure. The specific projects in hand are the enrichment of the methyl groups of methinine residues, the replacement of terminal residues of the chain, and the replacement of residues within the inner parts of the sequence.